1. Field of the Invention
The present invention relates to a novel and useful method and apparatus for rapidly and accurately measuring the concentration of a gaseous or volatile substance in a liquid. More particularly, it relates to and apparatus and method for measurement of the concentration of a gaseous or volatile substance in a liquid which can be suitably applied in industrial processes wherein measurement for an extremely long period of time and highly reliable results are required under conditions of violent flow and deep spots of the liquid.
2. Description of the Prior Art
A method and appararus measurement of the concentration of a gaseous or volatile substance is disclosed by U.S. Pat. No. 4,257,257 which is comprised of a sampling device (sampler) for sampling of a gaseous or volatile substance in a liquid using a liquid repellent porous partition tubing having continuous minute channels and a detector of the foregoing substance. In this U.S. Patent, a basic method and apparatus for the measurement of the concentration of the gaseous or volatile substance is taught, which comprises the steps of immersing a liquid repellent porous partition tubing having continuous minute channels extending through the wall of the tubing into a liquid to be tested, connecting a carrier gas supply pipe to one end of the tubing and a carrier gas discharge pipe to the other end, passing a given amount of carrier gas through the tubing, leading the carrier gas to a detector, detecting the quantity of the gaseous or volatile substance permeating through the tubing wall from the liquid and diffusing into the carrier gas, thereby measuring the concentration of the substance in the liquid continuously or intermittently.
The invention revealed by said U.S. Patent is only concerned with a fundamental principle of the measurement method, materials of the porous partition tubing employed and a basic and general method and apparatus. In this sense, it may be safely said that it is not always suitable for a practical industrial scale, but feasible tentatively on a beaker scale in the laboratory. That is, it is usable under relatively mild measurement conditions where properties of the liquid to be tested are suited to the tubing material, pressure imposed on the tubing is relatively small since, on a beaker scale, flow of the liquid is considerably mild, depth of the liquid is shallow, the measurement is effected in an open system and continuous measurement time is fairly short, for example, several tens of hours, and the like. As a matter of fact, when the measurement method and apparatus based on the principle as aforesaid is applied on an industrial scale to, for example, a fermentor used in cultivation processes of microorganisms, various problems arise and have to be solved. More concretely, even with, for example, a fermentor of microorganisms in which the measuring conditions are considered to be relatively mild, various problems are encountered when the foregoing method and apparatus for measuring gaseous or volatile substances in liquids employing a porous partition tubing having liquid repellency and continuous minute channels is adopted.
(1) The culture broth, in which various nutrients including predominantly inorganic salts and dissolved materials, shows an interfacial tension in the neighborhood of 40 dyne per cm even at a standard cultivation temperature of 30.degree. C., and sometimes shows as low as 37 dyne per cm, depending on the kind of the desired microorganisms.
(2) As a rule, in a practical process the volume of the liquid tested i.e., culture broth is great, the depth of the liquid sometimes 7 or 8 m, depending on the shape of the fermentor and the value of gas hold up. Accordingly high pressure is imposed on the sampler.
(3) Microorganisms are generally cultivated batchwise, especially be fed-batch method in most cases. As a result, the amount of the liquid increases with the lapse of time, in consequence, the depth of the liquid also increases with time.
(4) The cultivation of microorganisms is carried out by two different ways. One is a so-called anaerobic cultivation requiring no oxygen and the other is a so-called aerobic cultivation necessitating oxygen. When obtaining products contained by cells, by-products by cells or products contained in cells, aerobic cultivation is commonly conducted. In this case, a great amount of oxygen or oxygen-containing gas i.e., air is allowed to jet out through a plurality of nozzles having a minute diameter from a lower portion of the fermentor to thus uniformly disperse air bubbles in the culture broth, while, if required, circulating the culture broth by mechanical agitation. Furthermore, to improve dissolution of oxygen into the culture broth, it is occasionally practiced to position a choking means to an air exhaust line from the fermentor, thereby maintaining the inside pressure of the fermentor at rather increased pressure by the operation of the choking means.
(5) Not only continuous cultivation, but batch cultivation usually undergoes along-period cultivation, occasionally continuous measurement being required over 100 hours.
In summing up the foregoing problems, it is said that the culture broth has no properties suitable for the porous tubing of liquid repellency and continuous minute channels, because of its low interfacial tension in particular. Even if the porous tubing having pore size and porosity adaptable for the interfacial tension is properly selected, when the porous tubing is positioned to the fermentor and the measurement is effected, considerably great liquid pressure is imposed on the tubing coming from the depth of the liquid and pressurizing operation of the entire cultivation system. For the reasons, the interfacial tension of the liquid and the liquid pressure permit the liquid to permeate through minute channels. That is, in fact, fatal to the method for measurement of gaseous or volatile substances in liquids by the use of the porous partition tubing having liquid repellency and continuous minute channels. On the other hand, when the porous tubing is selected which has pore size and porosity enough to prevent the liquid from permeating through the minute channels, resisting the pressure imposed on the tubing, the passageway of gaseous or volatile substances permeating from the liquid into the carrier gas i.e., the total amount of continuous minute channels, reduces correspondingly, thereby leading to a decreased amount of gaseous or volatile substances permeating, with a result that said gaseous or volatile substances in the carrier gas passing through the porous tubing and the liquid to be tested do not reach gas-liquid equilibrium. That is also an obstacle to the utilizaton of the foregoing method.
In an attempt to reach the gas-liquid equilibrium, the surface area of the porous tubing has to be provided adequately, which means an increase in diameter and length of the porous tubing. Hence, the portion of the sampler immersed in the liquid to be tested increases. That gives rise to a disadvantage that the porous tubing is liable to suffer from damage and undesirable influences due to flow of the liquid.
In addition. in application of the foregoing measurement method to the practical process, there are problems as set below;
(6) The cultivation process of microorganisms requires a high degree of sterilization. For this purpose, not only the fermentor per se, but detectors of various measuring devices should have all such a construction that liquids are difficult to remain and that washing and sterilization are feasible. Prior to cultivation they are subjected to strict sterilization. The sterilization is normally performed by two ways; sterilizaton by the use of chemicals and thermal sterilizaton. It is common that various detectors are positoned to the fermentor, then the entirety of the fermentor is sterilized at 120.degree. C. to 130.degree. C. by heating under pressure.
The sampler of gaseous volatile substances in liquids providing the liquid repellent porous tubing having continuous minute channels should have also such a construction as permitting no residue of liquids, facilitating washing and sterilization and standing sterilization by heating under pressure by the use of steam of 120.degree. C. to 130 C.
(7) Different from the measurement on a beaker scale, the sampler, one installed to the practical fermentor, can be neither inspected nor exchanged. In the culture broth of microorganisms, there are contained various impurities including adhesive substances, in particular. These and cells adhere to the surface of the porous tubing during the course of cultivation for a prolonged period of time and, what is worse, cause blocking of minute channels. As a result, a decrease in sensitivity of the measurement or impossibility of the measurement is unavoidable.
As in apparent from the foregoing, the measurement method and apparatus using a porous partition tubing having liquid repellency and continuous minute channels disclosed by the U.S. Pat. No. 4,257,257, when applied to the measurement of the concentration of gaseous or volatile substances in liquids on a practical industrial scale process, involves a variety of problems to be solved.
For instance, when gaseous or volatile substances in a culture broth having the interfacial tension of approximately 40 dyne per cm are measured using a porous tubing made of tetrafluoroethylene resin having an average pore size of about 0.45 .mu.m, a porosity of about 60%, a thickness of about 500 .mu.m, an inside diameter of about 3 mm and an outside diameter of about 4 mm, which is found to be most preferable in respect of a liquid repellent porous tubing having continuous minute channels and exhibiting a superior sensitivity and strength, the culture broth starts to permeate through the minute channels of the tubing in only several hours to several tens of hours after the commencement of measurement, when the depth of the liquid is about 3 m, the measurement being thus hindered. Moreover, the foregoing tubing is placed in a steam sterilization atmosphere elevated at 120.degree. C. to 130.degree. C., the porous tubing is wholly deformed by the outside pressure to thus collapse. Further, the inside of the porous tubing and a carrier gas discharge pipe is filled with steam permeated through the continuous minute channels of the porous tubing and water resulting from condensed steam. When stains or plugging occur, it is next to impossible, on an industrial process, to make inspection or replacement, as was stated earlier.